Experimental studies on microstructural and corrosion

characterisation of additively manufactured AISI 316 L

austenitic stainless steel

Abstract

Microstructure and pitting corrosion behaviour of additively manufactured wall of AISI 316L

was investigated. The results indicated that weld zone, including ferrite and austenite phases,

was mainly composed of columnar dendrites. No obvious re-passivation was shown in as-

welded and thermally aged samples. Pitting occurred in both the as-weld and heat treated

samples. However, the pitting potential was less in thermally aged sample signifying less

pitting resistance.

Keywords: Wire-Arc Additive Manufacturing; Corrosion, Microstructure, Stainless steel

Introduction

Austenitic stainless steel is widely used as the structural material in many fields for its

outstanding mechanical and corrosion properties. [1] Welding is extensively selected

to connect these structural materials, and then the microstructure variation exists in the

weld joint.[2] It is believed that the microstructure discrepancy results in different local

corrosion behaviour such as pitting corrosion and intergranular corrosion and stress

corrosion cracking during service.[3]

AISI 316 L austenitic stainless steel is used as implant materials to make such devices

as artificial joints, bone plates, stents and so on, thanks to its favourable combination

of mechanical properties, corrosion resistance, satisfactory biocompatibility and

relatively low cost compared with other metallic biomaterials [4].

However, after its implantation in human bodies, sometimes mechanical failure occurs

and tissue inflammation may happen, which makes safety become dissatisfactory and

insufficient for its biomedical applications [5]. Because of the high concentration of

Cl− and the temperature range of 36.7–37.2 °C, the human body fluid is considered a

severely corrosive environment and localized corrosions such as pitting, crevice

corrosion and fretting fatigue are probable to appear on AISI 316L steel [6].

Metal ions such as iron, chromium and especially nickel could be released and thus

cause toxicity to the body and deteriorate the AISI 316L biocompatibility [7].

Therefore, improvement of the biocompatibility of AISI 316L steel, especially its blood

compatibility when used in the vascular environment, can be beneficial to its safe use

in the human body.

The objective of this work is to investigate the microstructure and pitting corrosion

behaviour of AISI 316L stainless steel additively manufactured using WAAM.

Background and Literature Review

Literature review

According to the previous works, the weld joint is mainly composed of weld zone, heat-

affected zone (HAZ), and base metal. Normally, it is accepted that weld zone possesses

a duplex structure including ferrite and austenite phases and the ferrite in the weld zone

reduces the hot cracking susceptible of the weld joint, while Kwok [8] also found that

the presence of ferrite in laser-welded SS304 stainless steel joint contributed to a

decrease in pitting corrosion resistance. The HAZ, caused by thermal cycle during

welding process, exhibits to be more susceptible to pitting corrosion compared with

base metal. Second phase precipitation, recrystallization, and residual stress are

proposed as the accelerating factors for pitting corrosion resistance, degradation of

HAZ and it is reported that HAZ is the most sensitive zone for pitting corrosion.

However, Lu [9] found that weld zone had a lower breakdown pitting potential than

HAZ under gas tungsten arc welding process, confirming the existence of pitting

corrosion performance discrepancy of weld zone under different welding methods and

conditions.

Stainless steel

Stainless steel is an alloy of iron and several other elements (such as nickel, chromium,

molybdenum, and carbon) and due to these elements it is more resistant to corrosion

than plain iron or steel (which is simply iron and carbon).These stainless steel elements,

such as nickel, chromium, and other additives, give it a passive oxide layer that resists

the formation of rust and creates a shiny, reflective surface. This passive layer has the

unique ability to repair itself. The shiny surface of stainless steel is very difficult to

tarnish compared to plain steel, hence why it is called “stainless” steel. It is considered

environmentally neutral and inert that is why it has longevity because it does not leach

compounds that could modify its composition when in contact with elements like water

and air. The four grades of stainless steel have been classified according to their

material properties and welding requirements:

 Austenitic stainless steels

 Ferritic stainless steels

 Martensitic stainless steels

 Duplex stainless steels

Austenitic stainless steel

Austenitic stainless steels has a faced-centered cubic (FCC) crystal structure and a grain

structure consisting of austenite. It has a Cr-level of 16 - 25 % and an addition of up to

35 % Ni to stabilize the austenitic structure. Since so much Ni is added it also makes

the austenitic stainless steels more expensive than ferritic stainless steels. They are non-

magnetic and have a good formability and weldability. The temperature use-span is

wide, from temperature below 123 K up to red-hot temperatures. Austenitic stainless

steels has a wide use and it often used in aircraft applications, food and dairy industry

and pulp and paper manufacturing.

Ferritic stainless steel

Ferritic stainless steels has a body-centred cubic (BCC) crystal structure and a grain

structure consisting of ferrite. The Cr-level spans from 10.5 % to greater than 25 %.

Their low cost made it grow in use and they are well suited for use as light-gauge sheets.

They are ferromagnetic, contains a low amount of carbon and are for some alloys poor

for welding in thicker-walled pieces due to the formation of brittle martensite. They

cannot be hardened by heat treatment, but strengthen by annealing. Ferritic stainless

steels has a wide use in automotive exhaust systems and kitchen applications

Martensitic stainless steel

Martensitic stainless steels have either a martensitic α ′ - or ǫ-phase. Martensite forms

at high cooling rates, when the transformation of austenite happens diffusionless. They

have a low corrosion resistance due to the low amount of alloying elements used to

keep the martensitic phase stable. This reduces the corrosion resistance, but they still

fill a use since they have very good mechanical properties such as high strength and

hardness. They are perfect for high-temperature applications where a strong and stable

material is needed.

Duplex stainless steel

Duplex stainless steel is the newest type of stainless steel alloy. It is a combination of

austenitic and ferritic structure to achieve a high strength. They have a high corrosion

resistance due to the amount of Cr being more than 20 %. They are less expensive than

austenitic stainless steels due to the low amount of Ni used. They are often used in -

100 – 300 ◦C applications where austenitic stainless steels have been used before and

where high strength is needed.

Interpretation of the Microstructure of Steels

Microstructure is the very small scale structure of a material, defined as the structure

of a prepared surface of material as revealed by an optical. The microstructure of a

materials can strongly influence physical properties such as strength, toughness,

ductility, hardness, corrosion resistance, high/low temperature behaviour or wear

resistance. These properties in turn govern the application of these materials in

industrial practice.

Figure 1-Lattice of iron and other alloying elements [10]

Steel properties come from their microstructural phases, small gaps between atoms,

called interstices, are where small elements like carbon and nitrogen fit. As the alloying

increases, the straining in the atomic lattice increases, requiring more force to deform

the work piece, thereby increasing the strength.

When a very small fraction of the interstices in between the iron lattice is occupied by

carbon atoms, this interstitial-free steel is said to have a microstructure of ferrite. Ferrite

has a body-centered cubic (BCC) crystal structure. Ferrite is a microstructural phase

that is soft, ductile, and similar to pure iron. There is a limit on how much carbon can

fit in the gaps in the ferrite structure: 0.02 percent carbon at 1,340 degrees F (725

degrees C), but dropping to 0.006 percent (60 PPM) carbon at room temperature.

The gaps are a little larger in a phase known as austenite, which has a face-centered

cubic (FCC) crystal structure. At around 2,100 degrees F (1,150 degrees C), up to 2

percent carbon can fit into the austenite microstructure.

As the steel slowly cools from this temperature and carbon is forced out of solution,

the austenite transforms into a combination of ferrite and another phase

called cementite, also known as iron carbide, which has the chemical composition of

C. The amount of cementite that forms is a function of how much carbon is in the

steel. Because ferrite cannot contain more than about 60 PPM carbon at room

temperature, the rest of the carbon winds up as cementite.

Unlike ferrite, cementite has the characteristics of a ceramic: very hard and brittle, with

low toughness and little resistance to crack initiation and propagation. The mixture of

ferrite and cementite is called pearlite, named because it looks like mother of pearl

under a microscope, with alternating layers of ferrite and cementite.

With faster cooling, different dynamics occur. Above a critical cooling, the excess

carbon of the FCC austenite does not have time to diffuse out of the crystal structure

and form cementite. Instead, the carbon is trapped in with the now nearly pure iron and

forced into the interstitial locations that are not large enough to accommodate the

carbon atoms. This distorts and strains the crystal matrix into a body-centered

tetragonal (BCT) structure, forming a hard phase called martensite.

At higher carbon levels, more carbon is frozen into the BCT structure, further straining

the crystal matrix. This is why the hardness of martensite increases with carbon level.

The volume of the BCT martensite structure is larger than that of the FCC austenite, so

the freshly transformed martensite is compressed by the surrounding matrix.

If martensite is heated, carbon has the opportunity to diffuse out from the BCT

structure, reducing the distortion of the crystal matrix, leading to decreased hardness

and increased toughness. This heat treatment produces a microstructure of ferrite and

iron carbide (Fe

C) called tempered martensite.

Bainite is another microstructure that can form when austenite is cooled. It typically

consists of a combination of ferrite, cementite, and retained austenite. Because the

cooling rate to form bainite is slower than the cooling rate needed to form martensite,

carbon has some opportunity to diffuse out of the FCC austenite, allowing for the

formation of BCC ferrite. Bainitic microstructures have the best balance of strength

and ductility. The cooling rate is fast enough to increase the strength, while the rounded

hard microstructural constituents are not as prone to crack initiation and propagation as

if they were flat and elongated.

Figure 2-Schematic diagram showing time-temperature transformation curve for plain

carbon eutectoid steel [10]

Weld metal solidification

During the solidification of a pure metal the solid-liquid interface is usually planar,

unless severe thermal undercooling is imposed. During the solidification of an alloy,

however, the solid-liquid interface and hence the mode of solidification can be planar,

cellular, or dendritic depending on the solidification condition and the material system

involved.

Figure 3-Basic solidification modes: (a) planar solidification, (b) cellular

solidification, (c) columnar dendritic solidification, (d) equiaxed dendritic

solidification [11]

Typical microstructures resulting from the cellular, columnar dendritic, and equiaxed

dendritic modes of solidification in alloys are shown below-

Figure 4-Nonplanar solidification structure in alloys. (a) Transverse section of a

cellularly solidified Pb–Sn alloy, (b) Columnar dendrites in a Ni alloy, (c) Equiaxed

dendrites of a Mg–Zn alloy, (d) Three-dimensional view of dendrites in a Ni-base

super-alloy [11]

Corrosion in materials

Types of corrosion

Galvanic Corrosion

Galvanic corrosion is also known as bimetallic corrosion, as the name suggests

two different types of metal, when two dissimilar metals kept together directly

or indirectly, in one of the metals corrosion rate occurs at a faster pace and the

other metal remains unaffected. There is basically a galvanic couple that forms

between the metals and one become anodic and the other cathodic. The main

factor which plays the role is the magnitude of the potential difference between

the metals.

High-temperature Corrosion

Generally above 500ºC high-temperature corrosion takes place. Although there

are many things that occur at high temperatures one of the main things is

oxidation. Because of this the loss in material changes in mechanical properties

because of the change in the microstructure of the alloy, and many more occurs.

Intergranular Corrosion

In this corrosion, grain boundaries are under attack, and carbides are formed,

further propagating for higher corrosion. The cause of this corrosion, in general,

is improper heat treatment. In austenitic stainless steel, chromium carbides can

become precipitated at the grain boundaries which reduces the individual

chromium concentration and makes boundaries prone to corrosion.

Localized Corrosion – Pitting

The most common form of localized corrosion is pitting corrosion. As the name

suggests it forms a hole of a small diameter that penetrates into the surface,

generally visible to naked eyes and the remaining surface is intact. The leading

causes of pitting corrosion are lack of passive film, high aggressive medium in

which the metal exists, poor coating application, and much more.

Localized Corrosion – Crevice

One of the highly penetrating types of localized corrosion is a crevice, which

occurs due to a small gap in the surface of the metal for example cracks, seams,

small spaces that occur during manufacturing, and much more.

Localised Corrosion – Filiform

Corrosion that occurs under the coated surface on the main metal. Occurs due

to the defects in the coating of the surface when some external elements attack

the main metal because of the defect in the protective layer.

Stress Corrosion Cracking (SCC)

The corrosion occurs due to the tensile, bending, and torsion stress generally

often at rapid high temperatures. The only prevention for SCC is to choose

proper fabrication with suitable material and suitable thermal and mechanical

stresses.

Electrochemical techniques

The condition of corrosion in a concrete or metal element is determined through

electrochemical corrosion testing. Electrochemical corrosion testing, which is based on

electrochemical theory, evaluates corrosion damage and, when possible, determines

corrosion rates. All corrosion is an electrochemical reaction involving oxidation and

reduction. To assess the corrosion characteristics of metals and metal components in

conjunction with various electrolyte solutions or soil conditions, controlled

electrochemical experimental approaches can be utilised. Each metal/solution system

has its own corrosion properties. The ASTM/NACE methods, Cyclic Polarization,

Impedance Spectroscopy, linear polarisation tests, electrochemical testing for hydrogen

permeation, real-time corrosion forecasts for materials, and others are among the

electrochemical techniques.

Cyclic Potentiodynamic Polarization (CPP)

CPP test was introduced for the first time in the 1960s. CPP measurements should be

carried out according to the defined ASTM standards (F2129, G61). It is widely used

to determine resistance to localized corrosion or degradation rate in a short time. Thus

this technique is applicable as a method for prediction of localized corrosion also

beneficial for alloys that are passivized spontaneously and underwent localized

corrosion.

General shape of CPP curve is as follows; after passing through the region of active

corrosion, the current density decreases to a critical potential, called the “Flade

potential” or “primary passivation potential”. This decrease is due to the formation of

the passive layer on metal surface.

The passive current density is the current density in the passive region. With further

increase in the potential in the passive region, a rapid rise in the anodic current can be

detected. This rise is due to either the evolution of oxygen by the decomposition of

water or breaking the passive film and localized corrosion. If the increase of current

density is due to the decomposition of water and evolution of oxygen gas, the region is

called “transpassive region”

Figure 5-Schematic illustration of CPP curve [12]

Corrosion control techniques

The following methods are used to protect metals against corrosion:

Selection of the right material of construction, Surface coating, use of Inhibitors, proper

equipment design and electrical protection

1. Barrier coatings

One of the easiest and cheapest ways to prevent corrosion is to use barrier

coatings like paint, plastic, or powder. Powders, including epoxy, nylon, and

urethane, adhere to the metal surface to create a thin film. Plastic and waxes are

often sprayed onto metal surfaces. Paint acts as a coating to protect the metal

surface from the electrochemical charge that comes from corrosive compounds.

Today’s paint systems are a combination of different paint layers that serve

different functions. The primer coat acts as an inhibitor, the intermediate coat adds

to the paint’s overall thickness, and the finish coat provides resistance to

environmental factors.

The biggest drawback with coatings is that they often need to be stripped and

reapplied. Coatings that aren’t applied properly can quickly fail and lead to

increased levels of corrosion. Coatings contain volatile organic compounds, which

make them hazardous to people and the environment.

2. Hot-dip galvanization

This corrosion prevention method involves dipping steel into molten zinc. The

iron in the steel reacts with the zinc to create a tightly bonded alloy coating which

serves as protection. The process has been around for more than 250 years and has

been used for corrosion protection of things like artistic sculptures and playground

equipment.

Unfortunately, galvanization can’t be done on-site, meaning companies must

pull equipment out of work to be treated. Some equipment may simply be too large

for the process, forcing companies to abandon the idea altogether. In addition, zinc

can chip or peel. And high exposure to environmental elements can speed up the

process of zinc wear, leading to increased maintenance. Lastly, the zinc fumes that

release from the galvanizing process are highly toxic.

3. Alloyed steel (stainless)

Alloyed steel is one of the most effective corrosion prevention methods around,

combining the properties of various metals to provide added strength and resistance

to the resulting product. Corrosion-resistant nickel, for example, combined with

oxidation-resistant chromium results in an alloy that can be used in oxidized and

reduced chemical environments. Different alloys provide resistance to different

conditions, giving companies greater flexibility.

Despite its effectiveness, alloyed steel is very expensive.

4. Cathodic protection

Cathodic protection protects by electrochemical means. To prevent corrosion,

the active sites on the metal surface are converted to passive sites by providing

electrons from another source, typically with galvanic anodes attached on or near

the surface. Metals used for anodes include aluminum, magnesium, or zinc.

While cathodic protection is highly effective, anodes get used up and need to

be checked and/or replaced often which can drive up costs of maintenance. They

also increase the weight of the attached structure and aren’t always effective in

high-resistivity environments.

Methodology and experimental details

Methodology

Figure 6- Flowchart showing methodology opted in the present work

Experimental details

The experimental details of the microstructure evaluation and corrosion examination

of WAAM manufactured AISI 316L welds is given as follows-

Microstructure Evaluation-

For carrying out the microstructural evaluation of the fabricated welds, samples were

cut from the fabricated walls and the surfaces were sanded and polished down up to

3000 grit of emery paper .These samples were etched using carpenter’s etchant. Each

sample was immersed in the etchant at 20

C and then water rinsed.

Table 1- Carpenter's etchant composition used in the present examination

Carpenters

Stainless

Steel Etch

FeCl

8.5 grams

Immersion

etching at

20 degrees

Celsius

For etching

duplex and

300 series

stainless

steels

CuCl

2.4 grams

Hydrochloric acid

122 ml

Nitric acid

6 ml

Ethanol

122 ml

An inverted type optical metallurgical microscope was used to examine microstructure

on etched samples.

Corrosion Examination-

Corrosion characterisation of the welds was done using a potentiostat (Gamry

Instruments, model: Reference 600) supported by Gamry framework software. For

conducting cyclic potentiodynamic polarization (CPP) test in a paracell at room

temperature, where three electrodes, namely a saturated calomel electrode (SCE) as a

reference electrode, graphite counter electrode and working electrode (weld test

sample), were used. Each sample was exposed to an electrolytic solution of 0.5 M

(sulfuric acid) + 0.5 M NaCl (sodium chloride).

Results and Discussions

A: Microstructural Studies of Welds

Weld metal microstructures of the test samples are presented in Figures 6(a) through

(b). The microstructure of the HAZ is characterised with dendrites. The weld zone is

solidified to form austenite + ferrite matrix in additively manufactured AISI 316L. The

figure 6(a) represents individual fusion boundaries of additively manufactured layers.

Each HAZ and fusion zone is clearly visible in the microstructure. The grains are

closely packed in the HAZ region and are loosely spaced in the weld zone.

Figure 7-Microstucture revealed using carpenters etchant

Figures 7(a) to 7(c) shows the microstructure of WAAM wall taken at different

positions. The figures reveal that in all the three locations austenite matrix was present

throughout the additively manufactured wall

Figure 8- Image showing microstructures of additively manufactured WAAM wall (a)

At the bottom-layer, (b) At the middle layer, (c) At the top layer

B: Corrosion behaviour

Pitting potential - The least positive current and voltage at which pits develop or grow

on a metallic surface. This is the electrochemical potential in a given environment

above which a corrosion pit initiates on a metallic surface

Corrosion potential - Corrosion potential is a mixed potential (also an open-circuit

potential or rest potential) at which the rate of anodic dissolution of the electrode equals

the rate of cathodic reactions and there is no net current flowing in or out of the

electrode.

Repassivation Potential - Pits repassivate below the repassivation potential, denoted

by E

CASE 1: As-Welded Condition (CPP)

Table 2-CPP test conditions for as-welded test sample

Initial E (V)

-0.25

Apex E (V)

1.5

Final E (V)

Forward Scan (mV/s)

Reverse Scan (mV/s)

Sample Period (s)

Apex I (mA/cm^2)

Density (g/cm^3)

7.805

Init. Delay Time (s)

300

Results

 Pitting potential - 325.6 mV

 Corrosion potential -210.1 mV

 Re-passivation Potential - NIL

Figure 9- Cyclic potentiodynamic polarisation test results for as-welded sample

Figure 10- Weld sample after CPP test (significant pits observed on the surface)

CASE 2: Thermally Aged Condition (CPP)

Thermal ageing condition: 750

C for 24 hrs and water quenched

Table 3-CPP test conditions for thermally aged test sample

Initial E (V)

-0.25

Apex E (V)

1.5

Final E (V)

Forward Scan (mV/s)

Reverse Scan (mV/s)

Sample Period (s)

Apex I (mA/cm^2)

Density (g/cm^3)

7.805

Init. Delay Time (s)

300

Results

 Pitting potential – [-190.1 mV]

 Corrosion potential 262.3 mV

 Re-passivation Potential - NIL

Figure 11-Cyclic potentiodynamic polarisation test results for thermally-aged sample

Figure 12-Thermally aged weld sample after CPP test (significant pits observed on

the surface)

Figure 13- Curve showing overlay plot of cases considered

Few noteworthy findings from the graph are-

 Percentage Difference Pitting potential - 41.6%

 Percentage Difference Corrosion potential - 24.8%

 Percentage Difference Re-passivation Potential - NIL

Figure 14-Image showing sample preparation (a) WAAM wall manufactured for

experimentation, (b) Samples cut for examination, (c) Polished sample

Figure 15- Equipment used in the present work (a) Muffle furnace used for thermally

ageing the samples, (b) Potentiostat used for conducting corrosion tests

Figure 16- (a) Precision balance used for measuring the proportions of the salt for

electrolyte formation, (b) Electrolytic solution made, (c) Experimental setup

Conclusions

The microstructure and pitting behaviour of AISI 316L stainless steel WAAM wall was

investigated. Microstructures in the additively manufactured wall was examined by

OM, the pitting corrosion of the as welded and heat treated samples of the stainless

steel wall was investigated by potentiodynamic polarization tests using the neutral

chloride electrolyte 17.5 gm NaCl in 500 mL solution. The results were obtained as

follows:

1. Weld zone, exhibited a dual structure including ferrite and austenite phases and

was composed of columnar dendrites primarily.

2. Compared with weld zone, grain refinement occurred in the HAZ.

3. A significant decrease of 41.6% and 24.8% in pitting potential and corrosion

potential after thermal aging of the specimen was observed. Higher pitting

potential indicates higher pitting resistance.

4. In both the specimen as-welded and thermally aged the re-passivation potential

was not observed.

5. In thermal aged specimens the amount of pits were significantly higher as

compared to as-welded condition.

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